This application is a U.S. National Phase Application of PCT International Application PCT/JP2012/003875 filed on Jun. 14, 2012, which claims priority to Japanese Patent Application No. 2011-144023 filed on Jun. 29, 2011. The disclosures of these applications including the specifications, the drawings, and the claims are hereby incorporated by reference in its entirety.
1. Technical Field
The present invention relates to fiber lasers using double-clad fibers, in particular, relates to the structure of a fiber laser capable of removing residual excitation light as unwanted light that is fed together with oscillation light from an optical fiber oscillator.
2. Background Art
For removing residual excitation light in double-clad fibers of a fiber laser, the following related arts have conventionally been suggested.
According to the first one of the related arts, an optical fiber protecting body is provided to protect a fusion splice at which the double-clad fibers are fusion spliced with each other. The protecting body above has an accommodating groove for accommodating the fusion splice therein and a supporting groove for supporting an optical-fiber coat section disposed close to the fusion splice. Besides, the protecting body has a heatsink for converting the light that leaks from the fusion splice into heat and then diffusing the heat into the outside. The fusion splice is covered with resin material so as to be fixed in the accommodating groove. The resin material is made of UV cured resin, allowing the light that leaks from the fusion splice to pass through (see patent literature 1, for example).
The second one of the related arts suggests an optical-fiber fusion splicing structure capable of removing residual excitation light trapped in optical fibers at a fusion splice that connects each end of two optical fibers. According to the structure, the fusion splice is linearly fixed and recoated with a resin whose refractive index is smaller than that of the clad or coating of optical fibers. Further, the resin is coated with a reinforcing sleeve. The fusion splice is thus retained (see patent literature 2, for example).
The conventional arts above, however, have the problems below.
According to the structure of the first related art, residual excitation light is removed by concentration. The UV cured resin covering the fusion splice generally has high refractive index. If residual excitation light to be removed concentrates at a part recoated with such a high refractive-index resin, the resin can be damaged by heat. The structure has insufficiency in consideration to cope with the problem above.
The structure of the second related art has the same problem with that of the first one. As is shown in an exemplary embodiment where an excitation light of 15 W is employed, the structure is suitable for low-output excitation. In the case of high-output excitation greater than several kilowatts, residual excitation light, which is removed at a part recoated with resin, concentrates in the periphery of the fusion splice, resulting in damage to the resin by heat. The structure, too, has insufficiency in consideration to cope with the problem above.